Flowmeters for measuring flow velocities and flow rates of fluids to be measured that are passing in fluid pipes are roughly divided into two types according to measurement principles.
A first type of flowmeters measures flow rates by making use of the fact that the processing amount of a fluid flowing through a fluid pipe changes, depending upon flowing direction. As this type of flowmeter, there is an orifice flowmeter. The orifice flowmeter measures flow rates by utilizing the fact that fluid pressure on upstream side of an orifice differs from that on the downstream side. This flow rate measuring method will be hereinafter referred to as “average value approximation.”
The second type of flowmeters is used for measuring flow rates of flows primarily in round pipes.
This type of flowmeters is adapted to measure a flow velocity at one point in the flow in a pipe, e.g., at a predetermined point on a pipe axis, and then a flow velocity profile configuration in the pipe is presumed from a theoretical value based on the obtained measurement value. The flow velocity profile configuration is integrated to determine a flow rate. This flow rate measurement method will be hereinafter referred to as “approximate integration.”
Meanwhile, some flowmeters are known as ultrasonic flowmeters for measuring the flow rates of fluids by applying ultrasonic waves to fluids to be measured.
Such ultrasonic flowmeters are roughly divided into a type adapted to measure the flow rates by the average value approximation method and a type adapted to measure the flow rates by the approximate integration.
The ultrasonic flowmeters employing the average value approximation measure the flow rates by determining an average velocity between two predetermined points by utilizing the fact that the time required for an ultrasonic pulse to travel between the two predetermined points differs by the flow velocity of a fluid, depending on whether the ultrasonic pulse advances towards the upstream side of the flow of the fluid or conversely towards the downstream side of the flow.
The ultrasonic flowmeters employing the approximate integration determines the velocity of a fluid to be measured at one point on the central axis of a pipe by utilizing the Doppler shift method thereby to measure the flow rate thereof on the basis of the determined velocity of the fluid, one of which has been disclosed in Japanese Patent Laid-open Publication No. HEI 6-294670. The ultrasonic flowmeter based on the approximate integration determines a flow velocity profile configuration from a theoretical value or an empirical rule and then performs integration. For instance, flow velocity profile in a laminar flow region in a pipe is represented by a parabola, so that the flow rates can be determined by using a fluid velocity measured on its central axis by using boundary conditions on a pipe wall. Strictly speaking, this theoretical solution applies to a flow in a steady state, and therefore, the ultrasonic flowmeter based on the approximate integration can be applied only to a flow of a steady state and cannot be used for the flow in non-steady state.
In general, the flow of a viscous fluid is widely known to be represented by Navier-Stokes equation (hereinafter referred to as “NS equation”). A conventional ultrasonic flowmeter determines flow rates by utilizing the knowledge of flow distribution with respect to a steady state, ignoring time derivative term of an NS equation. For this reason, if an object to be measured is a flow field (the flow field of a fluid) where the approximate integration does not hold due to time-dependent changes in the flow rate, then measurement accuracy may be significantly deteriorated or validity of measurement results may be damaged.
Such flow fields include, for example, a flow field in which a change time of a flow rate system is shorter than the time required for determining an average flow rate, or a flow field in which a flow has not yet fully developed. In the former case, the time derivative term of the NS equation does not reach zero, while in the latter case, one-dimensional approximation of the NS equation does not hold.
The conventional flowmeters are for performing flow rate measurement in steady states, so that measuring flow rates with sufficient accuracy requires, for example, an extremely long runway for stabilizing a flow on the upstream side of a measurement location. This requires time, cost and labor to provide piping. In addition, since the flowmeters are for measuring flow rates of flows in steady states, it has been difficult to measure flow rates of flows in non-steady states.
Furthermore, the conventional flowmeters are adapted to measure an average flow rate of a fluid passing in a closed pipe, such as a round pipe, making it impossible to measure local flow rates of larger flow systems. For instance, none of the flowmeters have been able to measure characteristic flow rates that vary with time in the vicinity of an inlet or outlet of a huge agitating tank.
The flow of a fluid to be measured in a flow field of a three-dimensional space is represented by a three-dimensional vector amount, while a conventional flowmeter measures a flow rate, presuming a one-dimensional flow in a pipe. For this reason, even in a closed pipe, if a flow is three-dimensional, then flow rate measurement accuracy extremely deteriorates or the measurement becomes impossible. For example, immediately following a bent pipe, such as an elbow pipe or a U-shaped inversion pipe, the flow of a fluid turns to be three-dimensional due to a centrifugal action. The conventional flowmeter installed at such a location will not be able to perform accurate flow measurement.
The present inventors have proposed, in the description of Japanese Patent Application No. HEI 10-272359, a Doppler type ultrasonic flowmeter that utilizes ultrasonic Doppler shifts, and permits precise, time-dependent, contactless measurement of flow rates even if fluids to be measured are in non-steady states.
The Doppler type ultrasonic flowmeter adopts a technique to thereby directly calculate the flow rate from an instantaneous flow velocity profile of the fluid to be measured in the fluid pipe, and it has been found to present high accuracy and responsiveness in measuring the flow rates of fluids to be measured.
The conventional Doppler type ultrasonic flowmeters are also required to permit measurement of flow rates of fluids to be measured in fluid pipes with ease and great versatility.
In order to smoothly measure the flow velocities of fluids to be measured in various types of fluid pipes by Doppler type ultrasonic flowmeters, it is necessary to secure sufficient ultrasonic transmission efficiency and to secure sufficient reflected wave S/N ratios for fluid pipes having various pipe wall thicknesses.
In the conventional Doppler type ultrasonic flowmeters, the ultrasonic transmission characteristics of a metal wall of the fluid pipe are checked by changing the thickness of the metal wall so as to set an optimum thickness of the fluid pipe.
However, application of the Doppler type ultrasonic flowmeters to an actual equipment makes it impossible to change the thickness of fluid pipes, and ultrasonic flowmeters having optimum ultrasonic transmission characteristics for each type of the fluid pipes must be prepared, exhibiting poor versatility.
The present invention has been made, considering the circumstances described above, and it is a primary object of the present invention to provide a highly versatile Doppler type ultrasonic flowmeter that permits simple, easy, contactless and accurate measurement of flow rates of fluids to be measured in various fluid pipes.
Another object of the present invention is to provide a Doppler type ultrasonic flowmeter that automatically selects an optimum ultrasonic frequency or an optimum ultrasonic incident angle that causes a resonant transmission phenomenon to take place with respect to various wall thicknesses of fluid pipes so as to permit precise and accurate measurement of flow rates of fluids to be measured by utilizing ultrasonic Doppler shifts.
A further object of the present invention is to provide a Doppler type ultrasonic flowmeter that permits accurate and precise measurement of flow rates even of opaque or translucent fluids to which optical flow rate measurement methods cannot be applied.
A still further object of the present invention is to provide a Doppler type ultrasonic flowmeter that permits precise and accurate measurement of fluids to be measured in fluid pipes even if swirling flows or flows not parallel to pipes are produced in fluid pipes.